Method for autonomous handoff in a wireless communication system

ABSTRACT

A method and apparatus for handoff to a rescue channel is introduced without the need to send and receive handoff control messages. In the preferred embodiment of the present invention, coordination of the rescue procedure at the infrastructure and mobile station is provided first by disabling the mobile transmitter followed by subsequent detection of signal loss and frame erasures at the serving cells, then by enabling the mobile transmitter followed by subsequent detection of signal by a rescue cell, and finally by enabling the rescue cell channel transmitter followed by subsequent detection and reception of signal and frames by the mobile.

FIELD OF THE INVENTION

[0001] The present invention relates generally to wirelesscommunications systems and, more particularly, to a method forautonomous handoff from a first fixed communication unit to a secondfixed communication unit in a wireless communication system.

BACKGROUND OF THE INVENTION

[0002] In a cellular communication system, signaling and voice or datacommunications between mobile communication units and fixedcommunication units (such as the infrastructure which is composed ofcells) are carried over an RF channel. In a TDMA system such as GSM, theRF channel is an RF carrier and timeslot combination. In a CDMA systemsuch as described by IS-95, IS-2000, or WCDMA, the RF channel is an RFcarrier with signals from many communications independently spread withthe use of orthogonal Walsh codes and other unique identifiers. As amobile unit moves through a cellular system, it becomes necessary tosupport a handoff of the mobile unit from the first fixed communicationunit to a second fixed communication unit. In the case of CDMA, thishandoff might be a soft handoff that allows the first fixedcommunication unit to remain connected with the mobile unit along withthe second fixed communication unit. Soft handoff then permits cell sitediversity to improve the communication paths by combining the bestsignals from each fixed communication unit connection.

[0003] Cellular systems have always been faced with the challenge ofreducing or eliminating dropped calls. However, in aninterference-limited environment, when the traffic channel degrades, theability for the mobile and the infrastructure to exchange signalingmessages for the purpose of handover is severely hampered. If thismessage exchange cannot take place, then the necessary handover cannotoccur to move the mobile out of its current degraded traffic channelcondition. This ultimately results in a dropped call.

[0004] A partial solution to this problem is described in U.S. Pat. No.5.913.167. METHOD FOR TRANSFERRING A COMMUNICATION LINK IN A WIRELESSCOMMUNICATION SYSTEM, by Bonta, et al. The '167 patent provides themobile with list of reserved channels that can be used for handoff fromthe first fixed communication unit to the second fixed communicationunit when communication between the mobile unit and a first fixedcommunication unit is interrupted. Although the '167 patent provides apartial solution to this problem, no mechanism exists to insure that themobile unit and the first fixed communication unit are time coordinatedto perform what amounts to a blind handoff. If the mobile unit and theinfrastructure are not time-coordinated, the mobile unit may attempt toperform a handoff to infrastructure equipment that is unprepared toaccept the handoff. In addition, the prior art does not provide amechanism for handling simultaneous communication breakdowns that wouldlead to simultaneous dropped calls. This is particularly a problem whentrying to efficiently utilize equipment resources. Therefore, a needexists for efficiently performing an autonomous handoff within awireless communication system that reduces the chances thatinfrastructure will be unprepared to accept the handoff when it occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005]FIG. 1 is a diagram of a wireless communication system inaccordance with the preferred embodiment of the present invention.

[0006]FIG. 2 is a flow chart of a mobile unit procedure to handle frameerrors in a CDMA system in accordance with the preferred embodiment ofthe present invention.

[0007]FIG. 3 is a flow chart of a mobile unit procedure to handle frameerrors in a CDMA system in accordance with the preferred embodiment ofthe present invention.

[0008]FIG. 4 is a flow chart of a mobile unit procedure to handle frameerrors in a CDMA system in accordance with the preferred embodiment ofthe present invention.

[0009]FIG. 5 is a flow chart of an infrastructure procedure to handleframe errors in a CDMA system in accordance with the preferredembodiment of the present invention.

[0010]FIG. 6 is a flow chart of an infrastructure procedure to handleframe errors in a CDMA system in accordance with the preferredembodiment of the present invention.

[0011]FIG. 7 is a flow chart of an infrastructure procedure to handleframe errors in a CDMA system in accordance with the preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0012] In order to address the above-mentioned needs, a method andapparatus for handoff to a rescue channel is introduced without the needto send and receive handoff control messages. In the preferredembodiment of the present invention, coordination of the rescueprocedure at the infrastructure and mobile station is provided first bydisabling the mobile transmitter followed by subsequent detection ofsignal loss and frame erasures at the serving cells, then by enablingthe mobile transmitter followed by subsequent detection of signal by arescue cell, and finally by enabling the rescue cell channel transmitterfollowed by subsequent detection and reception of signal and frames bythe mobile.

[0013] To efficiently support simultaneous rescue operations formultiple mobiles, the infrastructure will handle simultaneous rendezvousactions in series. Thus, only one rescue channel needs to be reservedper cell. Mobile units having simultaneous breakdowns in communicationwill attempt to receive traffic/signaling frames utilizing the sameWalsh code. However, only one of the mobiles will actually be able toreceive the traffic/signaling frames without frame errors since theframes are spread based on a unique spreading code identified with themobile station. Once the infrastructure has successfully performed arendezvous with the first mobile, it will send a handover command (e.g.handoff direction message) to it to move to a normal traffic channelWalsh code. Meanwhile, the other (second) mobile will continue waitingon the reserved rescue channel. The infrastructure will again beginspreading the appropriate traffic based on the reserved spreading codeand PN sequences unique to the second mobile. This will enable thesecond mobile to begin receiving good traffic frames so that it too canbe handed off to a normal traffic channel spreading code.

[0014] With the inclusion of a coordination procedure for a rescueoperation that enables a handoff to a rescue channel without the need tosend and receive handoff control messages, the probability of asuccessful handoff is greatly increased. Specifically, by firstdisabling the mobile transmitter with subsequent detection of signalloss and frame erasures at the serving cells, then by enabling themobile transmitter with subsequent detection of signal by a rescue cell,and finally by enabling the rescue cell channel transmitter withsubsequent detection and reception of signal and frames by the mobile,the mobile will not hand off before infrastructure is ready.Additionally, by handling multiple mobile rescue operations in a serialmanner only one rescue channel per cell is needed. Most importantly,this rescue channel procedure will greatly reduce the occurrence ofdropped calls by performing a handoff that would normally fail when thetraditional handoff control message are not able to be exchanged betweenthe mobile and the infrastructure.

[0015] The rescue procedure is enhanced by initially attempting thetransmission at maximum power an unacknowledged message from the mobilecontaining the current pilot channel signal strengths measured from thestrongest n cells (subsequently this message will be referred to as anemergency PSMM). If this message is received prior to starting therescue procedure, the information from this message will aid inselecting the appropriate rescue cell. To allow demodulation at therescue cells it is necessary to provide them the current PN sequencestates and spreading codes used to support the mobile channels.Specifically, for IS95 or IS2000 it is necessary to provide the currentlong code mask and offset into the short code PN sequence to the rescuecells. When the BSC or similar entity determines that all prior cells inthe infrastructure's active set (i.e. set of previous serving cells thatdo not include potential “rescue cells”) have fallen out of lock withthe mobile, it will instruct other cells (the potential “rescue cells”)from the current neighbor list (common to mobile station andinfrastructure) to monitor the mobile given its current long code maskand its offset into the short code PN sequence.

[0016] If the emergency PSMM is received, the BSC can minimize thenumber of “rescue cells” involved in saving this call. Otherwise, in thepreferred embodiment the BSC must determine which cells to use forrendezvous based on available information such as current neighbor listinformation and the last received PSMM. In an alternate embodiment,information on the proximity of the serving cells to the potentialrescue cells in the neighbor list is also used.

[0017] In the preferred embodiment of the rescue procedure, when arescue cell is able to lock a finger on the mobile, then that rescuecell will key it's reserved channel transmitter and inform the BSC sothat that rescue cell is forced into the soft handoff connection withthe mobile. The mobile is given a handoff command to move it off of thereserved rescue channel onto a normal traffic channel Walsh code, thusmaking way for handling future call cases of breakdowns in communicationbetween the mobile and infrastructure.

[0018] Referring to the drawings, FIG. 1 illustrates a wirelesscommunication system 200, such as a code division multiple access (CDMA)digital radiotelephone system. Fixed communication units such as basetransceiver stations (BTS) 310, 311, 312, 313, 314, and 315, alsoreferred to as cells, communicate with a mobile station 330, alsoreferred to as a mobile unit, operating within area 321. Areas 320, 321,322 and 325 are served by BTSs 310, 311, 312 and 315 respectively, whileareas 323 and 324 are served by BTSs 313 and 314. BTSs 310, 311, 312 and315 are coupled to a base station controller (BSC), 351, which includes,among other things, a transcoder (XCDR) 340, a processor 260, and amemory 270, and which is in turn coupled to a mobile switching center(MSC) 350, also including a processor 260, and a memory 270. Similarly,BTSs 313 and 314 are coupled to BSC 352 which includes, among otherthings, a XCDR 340, a processor 260, and a memory 270, and which in turnis also coupled to MSC 350. BSCs 351 and 352 may be referred to as abase station subsystem (BSS). A BSS may also be defined as a single BSCand its associated BTSs. MSC 350 is coupled to the public switchedtelephone network (PSTN) 450. The PSTN, MSC, and BSS are genericallyreferred to as the infrastructure.

[0019] Wireless communication between BTSs 310, 311, 312, 313, 314, and315 and mobile unit 330 occurs via radio frequency (RF) channels whichprovide physical paths over which digital communication signals such asvoice and data are transmitted. Base-to-mobile unit communications aresaid to occur on a forward link, while mobile-to-base unitcommunications are referred to as being on a reverse link.

[0020] As shown in FIG. 1, a communication signal, 361 has beentransmitted on the forward link channel such as a traffic channel, bybase station 311 to mobile station 330. Additionally, a communicationsignal 371 has been transmitted on a reverse link channel such as atraffic channel by mobile station 330 in response to communicationsignal 361 from base station 311.

[0021] Wireless communication system 200 provides a number of logicalchannels (not shown) that are separated into two categories, trafficchannels and signaling channels. The signaling channels are intended forcarrying signaling information for broadcast control, mobile unit accesscontrol, synchronization, and mobility management. For example inCellular System Remote unit-Base Station Compatibility Standard of theElectronic Industry Association/Telecommunications Industry AssociationInterim Standard 95 or 2000 (IS-95 or IS-2000), the pilot channel andsync channel are physical channels which provide time synchronizationwhile a third physical channel called the Paging channel is used forsignaling and broadcasts system overhead information as well as sendingpages. The pilot channel is used by all mobile units as a phasereference for demodulation as well as to establish initial timesynchronization with the base station. The pilot channel is also used asa measurement point by all mobile units for mobility managementdecisions. The traffic channels are intended for carrying encoded speechand data, but also support signaling for mobile station management. Themobile station management signaling on the traffic channel will eithersteal speech frames or will reduce the throughput of speech frames onthe traffic channel, so the frequency of these transmissions is verylow. There are several types of mobile station management signalingmessages that are exchanged between the mobile unit and the BSC (inIS-95 and IS-2000) such as the neighbor list update message, in-trafficsystem parameters message, pilot strength measurement message (PSMM),handoff direction message, and the handoff complete message. Similarsignaling is performed in WCDMA.

[0022] As is well known, mobile station management of handoff proceduresare essential to maintaining a mobile station connection with theinfrastructure as the mobile unit moves through wireless communicationsystem 200. Without support of handoff, the movement of a mobile unitcould result in a loss in signal strength between the mobile unit andthe BTS that provides a traffic channel to support communication. Forexample, if mobile unit 330 in coverage area 321 served by BTS 311,moves out of coverage are 321 into coverage area 322, then the requiredsignal strength would be insufficient to maintain communication betweenmobile unit 330 and BTS 311. However, if handoff procedures aresupported, the mobile unit 330 could be instructed to establishcommunication with a traffic channel on BTS 312 such that communicationsignal 362 would be transmitted on the forward link from BTS 312 tomobile 330, and communication signal 372 would be transmitted on thereverse link to BTS 312 from mobile 330. In CDMA systems, it isessential to support soft handoff procedures that enable more than oneBTS to be able to communicate with a mobile unit simultaneously. Amongother things, this enables cell site diversity so that in a rapidlychanging multi-path faded RF environment at any given instant in time,the BTS with the most favorable radio communication path with the mobileunit can be utilized to support the communication. For example, asmobile unit 330 is moving toward coverage area 322 from 321, a softhandoff procedure would enable both BTS 311 and BTS 312 to transmitforward link communication signals 361 and 362, respectively, to mobileunit 330, and BTS 311 and 312 could receive reverse link communicationsignals 371 and 372, respectively, from mobile unit 330. In thisconfiguration, the best forward link communication signal, 371 or 372from BTS 311 and BTS 312 respectively, would be selected by the mobilereceiver. Likewise, the best reverse link communication signal, 361 or362, would be selected by XCDR 340. In CDMA systems such as IS95 andIS2000, the mobile unit receives a neighbor list update message and anin-traffic system parameter message shortly after the call begins. Aneighbor list is a list of cells and associated pilot channels that amobile unit is required to periodically measure to determine whether thecell should be considered for handoff. Typically, these cells aregeographically proximate to the cells currently serving the mobile. Forexample, BTS 310, 312, 313, and 314 are adjacent to BTS 311, and wouldlikely be in the neighbor list for BTS 311. However, BTS 315 is notadjacent to BTS 311, so it might not be contained in the neighbor listfor BTS 311. The neighbor list update message provides the mobile unitwith information on the neighbor pilot channels it should measure todetermine when a handoff to the neighbor cell is necessary. For example,when mobile unit 330 began the call on a traffic channel associated withBTS 311, it received a neighbor list update message containing pilotchannel information for BTS 310, 312. 313, and 314. With thisinformation, mobile unit 330 will measure the strength of the pilotchannel associated with BTS 312 as the mobile unit moves from coveragearea 321 toward coverage 322. The in-traffic system parameter messageprovides the mobile with, among other things, thresholds such as T_Addand T_Drop to govern the handoff procedure. When the strength of thepilot channel associated with BTS 312 is measured above the T₁₃ Addthreshold, the mobile unit 330 will then be able to request that a softhandoff be established with BTS 312. This request is sent to the BSC 351with the use of the PSMM message. BSC 351 will then select a trafficchannel on BTS 312. Then BSC 351 will command BTS 312 to prepare thetraffic channel for handoff and BSC 351 will send a handoff directionmessage back to the mobile with the identity of the selected trafficchannel. Once mobile unit 330 receives the handoff direction message, itwill return a handoff complete message back to BSC 351. This willcomplete the process of establishing a soft handoff between mobile unit330 and BTS 311 and BTS 312. This handoff procedure is dependent on thesuccessful delivery of these messages identified as communicationsignals 361 and 371.

[0023] In order to manage the pilot channel information that the mobileunit receives in the neighbor list update message or in the handoffdirection message, it stores the information in one of 3 lists or sets;mobile unit's active set, candidate set, and neighbor set. The neighborset contains pilot information for neighbors that are not sufficientlystrong enough to be in a soft handoff connection. The mobile unit'sactive set contains pilot information for active soft handoffconnections. The candidate set contains neighbor pilot information forBTSs that are strong enough to be in a soft handoff connection, but areawaiting a handoff direction message to promote them to the mobileunit's active set. In this example, the pilot associated with BTS 312was in the candidate set after being measured by mobile unit 330 asbeing above the T_Add threshold, but prior to mobile unit 330 receivingthe handoff direction message from BSC 351.

[0024] In CDMA systems such as the wireless communication system 200.multiple subscribers may transmit at the same time. sharing the samecarrier frequency as the signaling control channel. To distinguishbetween different subscribers each signaling channel and traffic channelis multiplied by a unique pseudo-noise (PN) code that is a combinationof a long PN code, a short PN code, and also a Walsh code for theforward link. When the received signal containing this PN code iscorrelated (or despread) by the receiver with a unique mobile unit PNcode, it enables the desired signal to be constructively added while theundesired signals from other mobile units in the case of the reverselink or from other cells (BTSs) in the case of the forward link to beaveraged out as noise. Additionally, in the case of the forward link theother signals from the same serving BTS are orthogonal to the desiredsignal for a given demodulator due to the Walsh code properties and chipalignment of the same serving BTS signals. There are multiple Walshcodes associated with each BTS and these Walsh codes are shared betweenBTSs. At a BTS, each Walsh code is orthogonal with all others at thatBTS and thus provides a unique traffic channel identifier for each callconnection supported by the BTS. This enables the mobile unit to recoverits intended forward link transmission from a BTS. However, since Walshcodes are shared between BTSs and all mobile units are on the samecarrier frequency, it is necessary to have soft handoff connectionsbetween the mobile unit and the adjacent BTSs when the mobile unit is inan overlapping coverage region of the adjacent BTSs. Without the softhandoff connection, the mobile unit cannot obtain a diversity benefitfrom the signal transmitted from the other BTS covering the same regionand suffers instead from increased interference from this other BTS. Thelikelihood of a dropped call is thus significantly increased if there isno soft handoff in the overlapping coverage regions.

[0025] Even with the support of soft handoff, there are many times thatdegraded traffic channel conditions will interrupt the ability for themobile unit and the infrastructure to successfully perform a handoffprocedure utilizing communication signals 361 and 371 in aninterference-limited system. A common example occurs in an urbanenvironment. Typically, as mobile unit 330 moves from coverage area 321toward coverage area 322, the pilot strength of BTS 312 will begin toslowly rise. This would typically precipitate a soft handoff procedure,placing BTS 311 and BTS 312 in soft handoff with the mobile unit 330.However, in an urban environment, shadowing from tall buildings mayprevent the mobile unit 330 from measuring a rise in the strength of thepilot channel for BTS 312 until the mobile unit moves into a line ofsight with BTS 312 as a result of turning onto a street where BTS 312 islocated. This may cause a rapid increase in the received pilot channelstrength of BTS 312. If the increased pilot strength is sufficientlyclose to the pilot strength of BTS 311, then communication signal 361will be interrupted. At that point, mobile unit 330 will experienceconsecutive frame errors that will prevent the necessary soft handoffprocedure to be completed. The mobile unit 330 will be forced to dropthe communication link with BTS 311, and the call will be dropped.

[0026] To solve these problems, a rescue channels is introduced. In thepreferred embodiment of the present invention an agreed upon rendezvousor meeting place is communicated between the mobile and theinfrastructure at a time when communications are not hampered bydegraded traffic channel conditions. The meeting place is referred to asa rescue cell. This requires the reservation of a spare channel for eachcell that might be used as a rescue cell. Each cell in a mobile'sneighbor list may reserve one or more channels to handle potentialbreakdowns in communication between the mobile and infrastructure.

[0027] The mobile and infrastructure can re-establish a communicationvia one of these reserved channels, which in a CDMA system correspondsto a Walsh code. These Walsh codes are provided to the mobile under goodsignaling conditions as a part of the neighbor list update message. Withthese reserved Walsh codes, the mobile is now empowered to force one ormore of the strongest pilot Walsh codes into it's active set in theevent that communication is lost with the infrastructure (i.e. frameerrors on the forward link (downlink) cause the mobile to turn off ordekey its transmitter). The mobile will only force reserved Walsh codesto be active from cells that are valid candidates for soft handoff, butare not able to be added due to the lost communication with theinfrastructure. To enable the mobile to take this action of forcingthese reserved cells into the mobile unit's active set, a coordinatedset of actions must occur between the mobile unit and the infrastructurewithout dependence upon the ability to exchange handoff controlmessages.

[0028] As discussed above, a resource efficient method of coordinatingthese actions begins with the mobile attempting the transmission atmaximum power an unacknowledged emergency PSMM message containing thecurrent pilot channel signal strengths measured from the strongest ncells. If this message is received prior to starting the rescueprocedure, the information from this message will aid in selecting theappropriate rescue cell. This emergency PSMM is sent at the mobile'smaximum power to help improve the probability that the message isdelivered successfully and with haste. Note that the emergency PSMM canbe used prior to the dekey event when X number of consecutive erasures(where, in the preferred embodiment of the present invention, X is lessthan 12) are received to reduce the possibility of a dekey event. Thebenefit of early delivery of this message will enhance the ability ofthe infrastructure to select the minimal set of rescue cells toparticipate in the rendezvous.

[0029] As discussed previously, if the mobile unit and theinfrastructure are not time-coordinated, the mobile unit may attempt toperform a handoff to the infrastructure equipment that is unprepared toaccept the handoff. Thus, in the preferred embodiment of the presentinvention, the mobile disables its transmitter (i.e. dekeys) whenconsecutive frames are in error (typically 12 consecutive frames). Themobile also sets timers to allow the infrastructure to get prepared toaccept a handoff while it continues to search for a rescue cell with ameasured pilot signal strong enough to be a valid rescue cell torendezvous with. Subsequent to the mobile dekeying, the infrastructurecells that are currently serving the mobile with a soft handoffconnection will detect the mobile's transmitter dekeying when therespective fingers fall out of lock with the mobile (signal loss andframe erasures are symptoms of the fingers falling out of lock with themobile). This causes the serving cells to inform the coordinating pointwithin the infrastructure (BSC/XCDR) of the loss of communication withthe mobile. If the BSC determines that all prior cells in theinfrastructure's active set have fallen out of lock with the mobile, itwill instruct other cells from the mobile's current neighbor list tomonitor the mobile using its current long code mask and its offset intothe short code PN sequence. These cells are potential “rescue cells”. Ifthe emergency PSMM was received, the BSC can minimize the number of“rescue cells” involved in saving this call. Otherwise, the BSC mustdetermine which cells to use for rendezvous based on availableinformation. This could be done with a variety of methods. For example.the BSC may select rescue cells based on current neighbor listinformation and the last received PSMM. Or the BSC could infer whichcells should be used for rendezvous based on the current cells that areserving the mobile unit by selecting cells from the neighbor list thatare in close proximity to the current cells. Or the BSC could selectrescue cells based on historical data from previous dropped calls toprobabilistically support the selection of rescue cells. Another methodwould be to use a predefined list of rescue cells that are most likelyable to provide an adequate connection with the mobile, such as anoverlay cell. Some predetermined time after each rescue cell is selectedand is prepared to monitor for the reception of the mobile's signal, themobile's timers that were set to allow time for the infrastructure toprepare for handoff will expire. At that time, the mobile unit willre-enable (key) its transmitter and force the rescue cell into itsactive set. Subsequently, if one of the rescue cell's that is monitoringthe mobile is able to lock a finger on the mobile that is now keyed(transmitting), then that rescue cell will key it's reserved channeltransmitter and inform the BSC. The BSC will then enable the rescue cellto be forced into the soft handoff connection with the mobile. When themobile is able to lock a finger on the rescue cell and begin decodingframes from the infrastructure, the handoff to the rescue cell reservedchannel is complete. Finally, the mobile is given a handoff command tomove it off of the reserved rescue channel onto a normal traffic channelWalsh code, thus making way for handling future call cases of breakdownsin communication between the mobile and infrastructure.

[0030] Because time-coordination is performed prior to handing off to arescue channel, in the preferred embodiment of the present invention thechances of being able to complete a handoff is greatly increased. Moreparticularly, these coordination steps of disabling the mobiletransmitter with subsequent detection of signal loss and frame erasuresat the serving cells, followed by the enabling of the mobile transmitterwith subsequent detection of signal by a rescue cell, and finallyfollowed by enabling the rescue cell channel transmitter with subsequentdetection and reception of signal and frames by the mobile enable ahandoff to a rescue cell to occur without the need to send handoffcontrol messages. Practically speaking, the preferred embodiment of thepresent invention greatly reduces the probability that a call will dropwhen communications break down between a mobile and the infrastructure.This is accomplished by providing a coordinated rescue procedure as analternative to the traditional method to perform a handoff that useshandoff control messages embedded in traffic channel frames between themobile and infrastructure.

[0031] It is possible that more than one mobile unit may simultaneouslyhave a breakdown in communication in a coverage area served by the samecell or cells. In this situation, each mobile may need to rendezvous(i.e. set up a soft handoff connection) with the same rescue cell. Thereare two possible solutions for handling simultaneous breakdowns incommunication between two or more mobiles and the infrastructure wherethe desired rescue cell is the same. One solution requires reserving afixed number of Walsh codes per cell that are common to all mobiles whohave that cell in its neighbor list. This requires an arbitrationprocedure to be added to insure that the mobile and infrastructure arein harmony with each other regarding the actual rescue Walsh code beingused by a mobile. The arbitration is needed in the event that more thanone mobile tried to use the same reserved Walsh code at the same time.In this scenario, the infrastructure transmits traffic channel frames toeach of the mobiles on independent reserved Walsh codes. It would be upto the mobile to figure out which Walsh code was being used to transmitits traffic frames. This could be handled in different ways. In thepreferred embodiment of the present invention the mobile that continuedto received bad frames for a predefined period would automaticallyswitch to the next Walsh code reserved for that cell. In an alternateembodiment, a hashing algorithm based on the mobile's ESN is used toselect a Walsh code. In yet another embodiment, the mobile receives anindication from the rescue cell that the rescue cell is transmitting tothe mobile's own identity or the mobile's call identity using this Walshcode. This indication could be in the form of periodic messages thatreduce the forward link frame rate or as a field in the speech frame oras a mask of the speech frame. If the mobile did not receive thisindication within a fixed period of time, then the mobile wouldautomatically switch to an alternate reserved Walsh code for that cell.This procedure would continue until the mobile was able to find thecorrect Walsh code being used to rescue this call. The preferredsolution to the varied arbitration schemes just discussed requires theinfrastructure to control the simultaneous breakdowns in communication.In this solution, the infrastructure will handle the simultaneousrendezvous actions in series. In this case, only one rescue channelneeds to be reserved per rescue cell. Both mobiles will try to receivetraffic/signaling frames from the same Walsh code. However, only one ofthe mobiles will actually be able to receive the traffic/signalingframes without frame errors because the frames are spread based on thereserved Walsh code and scrambled based on the mobile's long code maskwhich is a function the mobile's ESN. Once the infrastructure hassuccessfully performed a rendezvous with the first mobile, it will senda handover command to it to move to a normal traffic channel Walsh code.Meanwhile, the other mobile will continue to wait on the reserved rescuechannel. Then when the infrastructure is ready, it will begin spreadingthe appropriate traffic based on the reserved Walsh code and scrambledbased on the mobile's long code mask which is a function the mobile'sESN. This will enable the second mobile to begin receiving good trafficframes so that it too can be handed off to a normal traffic channelWalsh code.

[0032]FIG. 2 and FIG. 5 show a prior-art procedure for handling abreakdown in communications between the mobile unit and theinfrastructure. FIG. 2 provides the procedure from the mobile unit'sperspective and FIG. 5 provides the procedure from the infrastructure'sperspective. Conventionally, after a call begins, a mobile unit 330 willreceive neighbor list update information (501) periodically (e.g. at thebeginning of the call and after each handoff). The mobile unit 330 willkeep track of forward link frame errors. If twelve consecutive (502)frame errors are detected, the mobile will turn off its transmitter andstart a fade timer (503). The fade timer typically is set to a period of5 seconds. Turning off the transmitter will disable handoff procedures,closed loop power control, and transmission of traffic data. Turning offthe transmitter will also have the effect of the infrastructure losingcommunication with the mobile unit, resulting in detection of reverselink frame errors. If two consecutive (504) forward link frames arereceived without error, the mobile unit 330 will cancel the fade timer(507) and turn on its transmitter once again. Normal call processingwill resume (508) (i.e. handoff procedures, closed loop power control,and transmission of traffic data are enabled). If two consecutive (504)frames are not received without error before the fade timer expires(505), the mobile unit 330 will release (506) and will be recognized bythe infrastructure as a dropped call.

[0033] Similarly, in FIG. 5, after a call begins, the infrastructure,for example BSC 351, will keep track of reverse link frame errors. Ifframe errors (701) are detected, the infrastructure will start a fadetimer (702). The fade timer typically is set to a period of 5 seconds.If a reverse link frame (703) is received without error, theinfrastructure (BSC 351) will cancel the fade timer (706) and normalcall processing will resume (i.e. receipt of mobile signaling needed forhandoff procedures and closed loop power control, as well as receipt oftraffic data is enabled). If no reverse link frames (703) are receivedwithout error before the fade timer expires (704), the infrastructure(BSC 351) will release mobile unit 330 and this event will be recognizedby the infrastructure as a dropped call.

[0034] In the preferred embodiment illustrated in FIG. 3 and FIG. 4, thetreatment of forward link frame errors is handled uniquely to reduce thepossibility of dropping the call. After a call begins, a mobile unit 330will receive neighbor list update information (601) periodically (e.g.at the beginning of the call and after each handoff). This neighbor listupdate information will also contain reserved Walsh codes (one for eachequipped rescue cell). As an example, assume that mobile unit 330 isbeing served by BSC 351 and BTS 311. The mobile unit 330 will keep trackof forward link frame errors from BTS 311. If twelve consecutive (602)frame errors are detected, the mobile will turn off its transmitter andstart a fade timer (603) as well as a rescue delay timer. The fade timertypically is set to a period of 5 seconds. The rescue delay timer is setto a period long enough to enable the infrastructure to prepare itsresources for a possible rendezvous of mobile unit 330 with one of theinfrastructure's (BSC 351) rescue cells using a reserved Walsh code andassociated traffic channel. This rescue delay timer may be set to arange of values, but is typically set to 400 milliseconds. Turning offthe transmitter will disable handoff procedures, closed loop powercontrol, and transmission of traffic data. Turning off the transmitterwill also have the effect of the infrastructure losing communicationwith the mobile unit, resulting in detection of reverse link frameerrors. If two consecutive (604) forward link frames are receivedwithout error, the mobile unit 330 will cancel the fade timer (608) andrescue delay timer, and turn on its transmitter once again. With thetransmitter turned on, the mobile unit 330 must now send a PSMM (609) tothe infrastructure to let it know that the reserved rescue channels arenot being used at this point in time. Normal call processing will resume(610) (i.e. handoff procedures, closed loop power control, andtransmission of traffic data are enabled).

[0035] If two consecutive (604) frames have not been received withouterror and the rescue delay timer expires (605), the mobile unit 330 willstart a rescue allowed timer (606). The rescue allowed timer is set towait for the detection of an eligible neighbor pilot to rendezvous with(620). This timer can have a range of values, but would be less than 5seconds. Again, if two consecutive (607) frames have not been receivedwithout error and the fade timer (611) expires, the mobile unit 330 willrelease (612) from the call and will be recognized by the infrastructureas a dropped call. However, if the fade timer (611) has not expired, themobile unit will determine, based on the T₁₃ADD threshold, if anyneighbor cell pilot is an eligible candidate cell for promotion into themobile unit's active set (620). This determination is possible since themobile unit 330 is still measuring the strength of neighbor pilotchannels as long as it is engaged in the call. If there are no eligibleneighbor cells and the rescue allowed timer has not expired (621), themobile unit 330 will continue to wait for either two consecutive goodframes (607), expiration of the fade timer (611), detection of aneligible neighbor pilot (620), or expiration of the rescue allowed timer(621). If the rescue allowed timer expires, then the mobile againdetermines if two consecutive forward link frames have been received(622). If not and the fade timer (623) expires, the mobile will releaseand will be recognized by the infrastructure as a dropped call. If twoconsecutive good forward link frames are detected (622), the mobile unit330 must now send a PSMM (627) to the infrastructure to let it know thatthe reserved rescue channels are not being used at this point in time.Normal call processing will resume (628) (i.e. handoff procedures,closed loop power control, and transmission of traffic data areenabled). If there is at least one eligible neighbor cell pilot (620)detected before the rescue allowed timer expires (621), for example BTS312, then the mobile unit 330 will cancel the rescue allowed timer (629)and promote the eligible neighbor cell (BTS 312) or cells to the mobileunit's active set using the respective reserved Walsh code (630). Mobileunit 330 will then turn on its transmitter and start the rescue re-keytimer (631). When the mobile turns on its transmitter, the mobile willbegin transmitting as its last known closed loop power and begingradually increasing this transmit power up to the nominal levelpermitted. However, other power control procedures are possible such astransmitting at a fixed power level, transmitting at a delta powerincrement over the last known closed loop power, or some combination.Typically, the rescue re-key timer would be set to a period of 40milliseconds, but could be set up to 5 seconds. Next, the mobile unit330 will send a PSMM to the infrastructure to let it know which reservedrescue channels are being used by this mobile. This will enable theinfrastructure to release resources at rescue cells that the mobile unitis not including in the mobile unit's active set. At this time, themobile unit must wait to see if it can receive two consecutive goodframes now that these rescue cells are included in the active set (634).The mobile unit 330 will continue to wait for good frames until therescue re-key timer expires (633). If the rescue re-key timer expires,the mobile unit will turn off its transmitter and will continue waitingfor two consecutive good forward link frames (622), but will not allowthe rendezvous procedure to resume. If the rescue re-key timer does notexpire and two consecutive good forward link frames are received (634),then the mobile unit 330 will cancel the rescue re-key timer and thefade timer (635). Normal call processing will resume (636) (i.e. handoffprocedures, closed loop power control, and transmission of traffic dataare enabled). Mobile unit 330 will then be regarded as being in a softhandoff connection with both BTS 311 and BTS 312.

[0036] Continuing with the same example using references from FIGS. 1, 6and 7 of the preferred embodiment, the treatment of reverse link frameerrors is handled uniquely to reduce the possibility of dropping thecall. After a call begins for a mobile unit such as mobile unit 330, theinfrastructure, for example BSC 351, will keep track of reverse linkframe errors between mobile unit 330 and BTS 311. If frame errors (801)are detected, the infrastructure will start a fade timer (802). The fadetimer typically is set to a period of 5 seconds. If eight consecutiveframe errors are not detected (803), and a reverse link frame (813) isreceived without error, the infrastructure (BSC 351) will cancel thefade timer (810) and normal call processing will resume (i.e. receipt ofmobile signaling needed for handoff procedures and closed loop powercontrol, as well as receipt of traffic data is enabled). If eightconsecutive frame errors are detected (803), the infrastructure (BSC351) will select a rescue cell or cells (804) based on available staticand dynamic configuration information such as, but not limited to,neighbor lists, infrastructure's active set, last received PSMM,historical events, statistics, and known overlapping coverage cells. Inthis example, the infrastructure (BSC 351) may select BTS 312 and BTS313 as possible rescue cells. The infrastructure will also start arescue acquire timer (804) and instruct the selected rescue cells tomonitor mobile unit 330 in an attempt to acquire transmissions from themobile unit (805). The monitoring action uses a receiver at BTS 312 andBTS 313 to acquire the mobile transmission. The acquire action includesdetermining if the measured signal level is above a threshold anddetermining if frames can be successfully decoded, but may only includedetermining if the measured signal level is above a threshold. Therescue acquire timer is set to 1 second, but can be set to a range ofvalues between 20 milliseconds and 5 seconds. While the monitoringaction is occurring, the infrastructure continues to evaluate whether agood reverse link frame is detected by one of the BTSs in theinfrastructure's active set (BTS 311). If a good reverse link frame isreceived, the infrastructure tests whether a PSMM is received viacommunication signal 371. If a PSMM was received (807) or a PSMM was notreceived but rescue acquire timer expired (808), the infrastructure willknow that the rendezvous with the rescue cells (BTS 312 and BTS 313) hasfailed. In this case, the infrastructure will instruct the selectedrescue cells (BTS 312 and BTS 313) to stop monitoring for transmissionfrom mobile unit 330 (809). The infrastructure will then cancel (810)the fade timer and rescue acquire timer (if still running). Normal callprocessing will resume (i.e. receipt of mobile signaling needed forhandoff procedures and closed loop power control, as well as receipt oftraffic data is enabled). However as a part of the monitoring action, ifa good reverse link frame is not received (806) by one of the BTSs inthe infrastructure's active set (BTS 311), the infrastructure testswhether the fade timer has expired (811). If the fade timer has expired,the infrastructure will release (812) mobile unit 330 and this eventwill be recognized by the infrastructure as a dropped call. If the fadetimer has not expired, the infrastructure (BSC 351) checks to see if anyof the rescue cells (BTS 312 or BTS 313) were able to acquire mobileunit 330 (821). If none of the selected rescue cells (BTS 312 or BTS313) were able to acquire mobile unit 330, but the rescue acquire timerhas not expired (825), the infrastructure will return to waiting to seeif the next reverse link frame (806) can be detected by one of the BTSsin the infrastructure's active set (BTS 311). If no rescue cell (BTS 312or BTS 313) is able to acquire mobile unit 330 and the rescue acquiretimer expires, the infrastructure (BSC 351) will instruct (826) theselected rescue cells to stop monitoring for transmission from mobileunit 330. At this point, the infrastructure will no longer participatein a rendezvous with mobile unit 330. Instead, it will continue testing(827) to see if a good reverse link frame can be detected by one of thecells in the infrastructure's active set (BTS 311). If no reverse linkframe is received without error before the fade timer expires (828), theinfrastructure will release mobile unit 330 and this event will berecognized by the infrastructure as a dropped call (829). If theinfrastructure receives a reverse link frame by one of the BTSs in theinfrastructure's active set (BTS 311) without error, then theinfrastructure will then cancel (824) the fade timer. Normal callprocessing will resume with the support of the BTSs in theinfrastructure's (BSC 351) active set (BTS 311) (i.e. receipt of mobilesignaling needed for handoff procedures and closed loop power control,as well as receipt of traffic data is enabled). If one or more of theselected rescue cells (821) is able to acquire mobile unit 330, therescue cell (BTS 312) or cells will turn on its' transmitter (822), thusenabling closed loop power control to be resumed. The infrastructure(BSC 351) must now wait for the receipt of a PSMM (830) with anindication that rescue channels were used by mobile unit 330. If therescue acquire timer expires (831) before the PSMM is received, then theinfrastructure will turn off the rescue cell transmitter (BTS 312) andwill release rescue cell resources associated with this rescue attempt(832). The infrastructure will no longer participate in a rendezvouswith mobile unit 330. Instead, it will continue testing (827) to see ifa good reverse link frame can be detected by one of the cells in theinfrastructure's active set (BTS 311). If the PSMM is received (830),the infrastructure (BSC 351) will connect the traffic data (823)currently being supplied to the infrastructure's active set cells (BTS311) to the reserved rescue channel on the rescue cell (BTS 312) that isnow in the mobile unit 330 active set. This rescue cell is now added tothe infrastructure's active set, and the rendezvous is complete with thecancellation of the fade timer and rescue acquire timer. Normal callprocessing will resume with the support of the BTSs in theinfrastructure's (BSC 351) active set (BTS 311 and BTS 312) (i.e.receipt of mobile signaling needed for handoff procedures and closedloop power control, as well as receipt of traffic data is enabled). Inthe preferred embodiment, the infrastructure may now command the mobileunit 330 to move off of the reserved rescue channel onto a regulartraffic channel not normally reserved for a rendezvous. This isaccomplished with the use of the handoff direction message.

[0037] The preferred embodiment uses frame errors to detect the need tobegin a rendezvous, but other metrics such as bit errors, multiframeerrors, signaling discontinuity, or absence of measurable signal areother mechanisms to trigger the start of this rendezvous algorithm.

[0038] In an alternate embodiment, the mobile unit 330 sends a PSMM tothe infrastructure using up to its maximum transmit power. This occursjust prior to step 602, after 6 consecutive reverse link frame errorsare detected, or it may occur some time after step 602. If the PSMM issent after 602, then the mobile unit will need to briefly turn on itstransmitter long enough to send the PSMM. If the infrastructure (BSC351) is able to receive the PSMM prior to 804, then the infrastructurecan use this information to assist in selecting the rescue cell orcells.

[0039] In yet a further alternate embodiment, the mobile unit usesmechanisms other than a PSMM for informing the infrastructure of whichrescue cells it is using for rescue, such as sent in step (632) andreceived in step (830). For example, an indication of the rescue cellsbeing used by mobile unit 330 may be signaled repeatedly in the firstfew frames exchanged between mobile unit 330 and the rescue cell (BTS312) as a new message. Alternatively, the information could betransmitted on a separate channel such as an access burst on an accesschannel similar to the procedure used on a new call origination.

[0040] In an alternate embodiment, the mobile unit may not turn off itstransmitter as a result of detecting frame errors. Such is the case forTDMA systems such as Global System for Mobile Communication (GSM) oriDEN. In this case, the infrastructure needs some indication that themobile unit is intending to use a rescue channel when forward linkframes are not being received. This can be accomplished by eitherpermanently enabling the rescue channel to receive a mobile unit thatneeds a rescue cell, or it can be accomplished by having the mobile unittoggle its transmitter off. then back on again long enough for theinfrastructure to detect the fluctuation of signal from the mobile unitas a special signal of the need to rendezvous. This special signal maybe performed as a series of transmitter toggles through off and onstates to further qualify it as a special signal.

[0041] While the invention has been particularly shown and describedwith reference to a particular embodiment, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention. It is intended that such changes come within the scope of thefollowing claims.

1. A method for a mobile unit to facilitate autonomous handoff within acommunication system, the method comprising the steps of: communicatingwith a first base station; receiving an active set of base stations fromthe first base station; determining that communication has been lostwith the first base station; de-keying a transmitter to breakcommunication with the first base station, causing a second base stationto set up receiver resources to listen for the mobile unit; adding asecond base station to the active set of base stations without beinginstructed to do so by the first base station; and reactivating thetransmitter causing the second base station to communicate to the mobileunit.
 2. A method comprising the steps of: determining that a break incommunication has occurred between a remote unit and a plurality of basestations, wherein the break in communication was caused by a mobile unitde-keying its transmitter; based on the determination, determining arescue cell; and instructing the rescue cell to monitor transmission ofthe remote unit.
 3. The method of claim 2 wherein the step ofdetermining the plurality of rescue cells comprises the step ofdetermining the plurality of rescue cells from a message received by theremote unit.
 4. The method of claim 2 wherein the step of determiningthe plurality of rescue cells comprises the step of determining theplurality of rescue cells from pilot strength measurement informationreceived by the remote unit.
 5. The method of claim 2 wherein the stepof determining the plurality of rescue cells comprises the step ofdetermining the plurality of rescue cells from historical data ofprevious dropped calls.
 6. The method of claim 2 wherein the step ofdetermining the plurality of rescue cells comprises the step ofdetermining the plurality of rescue cells from the remote unit'sneighbor list information.
 7. The method of claim 2 wherein the step ofdetermining the plurality of rescue cells comprises the step ofdetermining the plurality of rescue cells based on proximity to aserving base station.
 8. The method of claim 2 wherein the step ofdetermining the plurality of rescue cells comprises the step ofdetermining the plurality of rescue cells based on whether the cellsprovide an overlap in coverage with a serving base station.
 9. Themethod of claim 2 wherein the step of determining that the break incommunication has occurred between the remote unit and a plurality ofbase stations comprises the step of determining whether a signal levelof a mobile unit falls below a first threshold at a serving basestation.
 10. A mobile unit comprising: means for determining that a poorcommunication exists between the mobile unit and a base station; atransmitter coupled to the means for determining, the transmitterde-keying to break communication with the base station, causing a rescuebase station to set up receiver resources to listen for the mobile unitand after a predetermined period of time, reactivates causing the rescuebase station to enable its transmitter to communicate to the mobileunit.
 11. The mobile unit of claim 10 wherein the means for determiningthat a poor communication exists between the mobile unit and the basestation comprises means for determining means for determining that apredetermined amount of bad frames have been received from the basestation.